1. Field of the Invention
This invention relates to the field of physical measurements, with particular application to the field of clinical thermometry.
2. Prior Art
The traditional mercury in glass termometer is rapidly being replaced by electronic thermometers in the field of clinical thermometry. There are two basic reasons for the preference for electronic thermometers over mercury in glass, namely, protection against cross infection, and speed of operation. Up to the present time only a moderate improvement in speed of operation has been realized. In order to protect against cross infection with current designs of electronic thermometers, it has been found necessary to use a disposable sheath to cover the sensor, since the sensors used are too costly to be considered disposable. Unfortunately, this combination results in a sensing system which takes several minutes to come into temperature equilibrium with the human mouth. No fast response sensor has yet been devised which is sufficiently inexpensive to be considered disposable, and yet is accurate enough to be used in a clinical thermometer for measuring human body temperatures. A sensor rugged enough to withstand continuous use with its disposable sheath presents a thermal system with an unavoidably long thermal time constant.
Several methods have been devised to speed the response of electronic thermometers which use long time constant sensor systems. Most common is the use of anticipation based on Newton's law of cooling. Using such anticipation circuits, practical electronic thermometers have been made which indicate the temperature being measured well before the sensor reaches its equilibrium temperature. These systems, while representing a substantial improvement over systems without anticipation, still result in a 30 to 45 second measurement cycle. Movement of the sensor in the mouth during this 30 to 45 second time period can cause inaccuracy or can cause the measurement time to be increased, or both.
To be useful, clinical thermometers must have an accuracy of plus or minus 10.2.degree. F. when measuring temperatures in the region of the normal human body temperature of 98.6.degree. F. An accuracy of plus or minus 10.1.degree. F. would be preferable, if attainable.
Bare bead thermocouples can be made inexpensively enough to be considered disposable from a clinical thermometer standpoint, and with sufficiently low mass to have rapid response, but unfortunately, the best accuracy obtainable from probes made of premium wire is probably plus or minus 10.5.degree. F. or worse. This accuracy is not adequate for clinical thermometers. Thermocouple probes have thus not been successful in clinical thermometers.
It is therefore an object of the present invention, in one of its aspects, to provide an electronic thermometer which utilizes disposable thermocouple probes, and which has adequate accuracy for clinical use.